I have no idea whether the following will work on a Mac.
It is generally not compatible with an IE browser either, but only with Firefox, Opera or Mozilla Suite.
Check the FAQs at the bottom of the page for more info. on Pornotube download specifics.

Gold Member

If you will get into the 21st century and use firefox (assuming you're not already doing so) then you can add some extensions to your firefox browser called video downloader and downloader helper that will allow you to save these flash movies. Once you have them on your system, you'll need a .flv viewer to use them. There isnt much choice on a mac for a .flv viewer, but I use flv viewer for os x. Good luck.

Gold Member

For flv files downloaded from Pornotube I've found a nifty freeware program called Super v2006.build.19 (by eRightSoft) that can convert their flv files to other formats. (It also converts a variety formats as well).

It doesn't seem to work with Youtube or Xtube flv files however (I get error messages). Maybe there's something I'm supposed to do that I haven't figured out yet.

TubeSock grabs YouTube videos from the web and copies them to your video iPod, Mac, or PlayStation Portable. TubeSock knows how to convert the video using the codecs and bitrates best for each device. It can even add the video to iTunes for you.​

The files on the other sites are the same format. This should work but I haven't tried it.

GetTubeVideo allows you to download videos from YouTube and XTube like sites. All downloaded videos can be converted by GetTubeVideo to other video formats including 3GP, ASF, AVI, FLV, MOV, MP4, MPEG and WMV.